1. Field of the Invention
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent locking of the wheels.
2. Description of the Prior Art
An anti-skid control apparatus for a vehicle braking system is known that includes a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, the fluid pressure control valve device receiving control signals from a control unit that measures how close the wheel is to skidding so as to control brake fluid pressure to the wheel cylinder.
When a fluid pressure control valve is provided for each of four wheels, with independent control of the fluid pressure, control is no problem. When a fluid pressure control valve device is provided for each of two front wheels and for both rear wheels in common, control is no problem. In the latter case, the one common fluid pressure control valve device is controlled on the basis of the lower of the speeds of the rear wheels.
However, the above case require three or four fluid pressure control valve devices. Accordingly, the whole anti-skid control apparatus is large, very heavy and expensive.
Accordingly, to provide a relatively small and economical anti-skid control apparatus for a vehicle braking system that can minimize the chance of locking of rear wheels, this applicant previously proposed an anti-skid control apparatus for a vehicle braking system that included fluid pressure generating means to which two brake conduit circuits were connected, a fluid pressure control valve for controlling fluid pressures of wheel cylinders of wheels connected to one of the two brake conduit circuits, and a valve apparatus for controlling fluid pressures of wheel cylinders of wheels connected to the other of the two brake conduit circuits. The valve apparatus comprised a casing having first and second input ports, an output port, a piston, a control chamber and a volume chamber formed at both sides of the piston. A valve was arraned between the first input port and the volume chamber, the first input port being connected to one fluid pressure generating chamber of the fluid pressure generating means, and the output port being connected to the wheel cylinders connected to the other of the two brake conduit circuits. When the fluid pressure control valve was operated, the piston was moved towards the control chamber to close the valve cutting off the first input port from the volume chamber and changing the volume of the volume chamber with the movement of the piston to change the fluid pressure of the wheel cylinders connected to the other of the two brake conduit circuits.
However, in the apparatus described above, since the braking forces on all of the wheels connected to the two brake conduit circuits were controlled by the one fluid pressure control valve, the braking forces on the wheels which were not locked or had not yet tended to lock were decreased along with those of the wheels which are locked or tend to lock. Accordingly, the braking forces were reduced unnecessarily for the wheels which were not locked. The braking distance of the vehicle was lengthened.
Further, when the one fluid pressure control valve was mechanically locked or held at a brake relieving position or brake holding position, there is the danger that no braking force was imparted to any of the wheels, or that the braking force became dangerously low.